Method and materials for applying chromate conversion coatings on zinciferous surfaces



United States Patent 3,136,685 BEECH-I01) AND MATERIALS FOR APPLYENG CHRQMATE CDNVERSION COATINGS 0N ZINCIFEROUS SURFACES George F. Qtto, Greland, Pa., assignor to Amchem Products, Ina, Ambler, P2,, a corporation of Delaware No Drawing. Filed June 4, 1963, Ser. No. 235,222 17 Claims. (Cl. 148-62) This invention relates to the art of coating metal surfaces and is particularly concerned with the application of a paint-bonding, chromate conversion coating to surfaces of zinc or alloys thereof wherein zinc is the principal ingredient. In the following disclosure use of the expression zinc surfaces is to be understood as applying to such alloys as well as to hot-dipped galvanized surfaces, to electro-deposited zinc coatings and to essentially pure zinc surfaces.

The production on zinc surfaces of chromate conversion coatings possessing the desired blend of corrosion resistance and paint-bonding qualities has become increasingly diflicult where certain advanced techniques are employed. For example, commercial strip lines are noW in operation whereby it is possible to process upwards of 1000 ft. of metal per minute. Such increased production speeds have resulted in serious bottlenecks at the coating stage in strip installations by virtue of the fact that presently available chromate conversion coating solutions are not capable of producing sufliciently high quality coatings under such high-speed operations. While conventional chromate conversion coating solutions and techniques appear capable of coating zinc surfaces in very short periods of time, for example in 5 to 15 seconds, the coatings produced on such rapidly moving zinc surfaces are very often too powdery to form an adequate substrate for the application of siccative finishes. Moreover, it has been found that such coatings frequently contain soluble salts which are leached from the coating during subsequent exposure to humid environment and, as a result thereof, the coated metal surfaces give rise to serious blistering and loss of subsequently applied paint finishes.

Several attempts have been made to overcome this problem. One such attempt involved the use of a chromate conversion coating solution having dissolved therein trivalent as well as hexavalent chromium ions together with one or more of the following, namely sulfites, sulfates, nitrates and acetates. In high-speed production practice, however, solutions of this type failed to provide the required corrosion resistance and paint-bonding qualities on the rapidly moving zinc strip.

Another suggestion involved the use of a solution of pine and hexavalent chromium ions, which solution was impinged upon the zinc surfaces to be coated after such zinc surfaces had been pro-heated to a temperature well above the boiling point of the solvent contained in the coating solution. This technique necessitates a coating temperature well in excess of 94 C., and frequently as high as 177 C., and has not proven to be economically adaptable to high-speed industrial production techniques.

With the foregoing in mind the principal object of the present invention will be more fully understood and appreciated and may be said to reside in the provision of a method and material for rapidly applying to zinc surfaces a highly corrosion resistant and paint-bonding chromate conversion coating even at ordinary or living room temperatures.

A concomitant object is the provision of an aqueous acid chromate conversion coating solution which, over long periods of use, is capable of forming consistently high quality coatings on zinc surfaces.

Yet another object of this invention is the provision of an aqueous acid chromate conversion coating solution which will essentially completely react with zinc surfaces thereby leaving no water soluble salts thereon after drying even without the benefit of a final rinse.

How these and other objects and advantages are attained with the teachings of this invention will become apparent from a study of the following description and examples.

The present invention is based upon the discovery that an aqueous acid solution consisting essentially of:

(a) At least 3.0 grams/liter of zinc ion (calculated as (b) From 5.0 to grams/liter of hexavalent chromium ion (calculated as C10 (0) From 1 to 20 grams/liter of chloride ion (calculated at C1), and

(d) At least 1.0 gram/liter of a complex fluoride radical selected from the class consisting essentially of TiF BF, and SiF said solution being maintained between the pH values of 0.57 and 3.0, and being substantially free of alkali and alkaline earth metal ions, when applied to zinc surfaces will produce thereon, within from 5 to 30 seconds, a highly corrosion resistant and paint-bonding chromate conversion coating.

In preparing the coating solutions of this invention it is essential, in order to insure satisfactory operation thereof, that substantially no alkali or alkaline earth cations be included therein. It has been found that the presence of such cations, including ammonium ions, must be avoided if consistently satisfactory coatings are to be obtained, particularly where the coatings we dried upon the treated zinc surfaces without intermediate rinsing. By the term substantially no alkali or alkaline earth cations is meant such cations which might be added deliberately as in prior art coating processes. This term is not intended to, and does not, imply that the normal levels of cations found in the usual commercial water supplies must be eliminated prior to use in the coating solutions of this invention.

The zinc ion (calculated as Zn), as noted hereinabove, must be present in the chromate conversion coating solutions of this invention in an amount of at least 3.0 grams/ liter. If the minimum amount of zinc is permitted to fall below 3.0 grams/liter, the zinc surfaces being treated will be subjected to an etching action with very little, if any, coating being formed thereon. So far as an upper limit of zinc ion is concerned, there is no apparent lhnitation with respect to the coating ability of the solution. For example, where the amount of zinc present in the solution exceeds the stoichiometric quantity required for forming hexavalent chromium salts thereof, such excess amounts of zinc will then become associated with other available anions in the coating solution, i.e., chloride and complex fluoride ions. When the amount of zinc exceeds the quantity requ red to become stoichiometrically associated with all of the anions present in the coating solution; then there occurs a precipitation believed to be zinc chromate (ZnCrO upon the zinc surfaces being treated. However, this salt precipitation has not been found to be detrimental to coating quality, and is undesirable only from the viewpoint of excess chemical consumption. In general, it is preferred to maintain the zinc concentration at levels where this cation will be present in a quantity which does not exceed the stoichiometric amount needed to become associated with the anions dissolved in the acid coating solution.

So far as concerns introduction of zinc into the coating solutions of this invention, this cation may be added as the oxide, or as a salt of one of the essential anions, such as for example zinc chloride, zinc dichromate or zinc fluosilicate. An important consideration to be observed is that whatever salt of zinc is employed, the anion portion resistance (ASTM-B-117-61 thereof must not be in any Way detrimental to the coating reaction. Anions which have been found to be detrimental when added to the coating solutions of this invention include sulfate and simple fluoride. The former anion causes rapid loss of coating quality and weight with severe etching of the zinc articles being coated. The latter anion essentially completely prevents coating formation and severely etches the zinc surfaces being treated.

With respect to the hexavalent chromium ion concentration (calculated as Cr it is essential that this constituent be present in amounts ranging from 5.0 to 75 grams/liter. Where less than the minimum amount is employed, the corrosion resistance and paint adhesion qualities of the subsequently produced coatings will be found to be adversely affected. Conversely, the upper limit of hexavalent chromium is equally critical, and should not exceed 75 grams/liter in order to avoid the occurrence of blistering under subsequently applied painted films after exposure of the metal to humid atmospheres.

Hexavalent chromium ion is preferably added to the coating solutions of this invention in the form of chromic trioxide (CrO However, this coating anion may be introduced as a salt of zinc if desired.

The amount of chloride ion (calculated as Cl) which must be employed in the freshly prepared coating solutions of this invention, must, as noted hereinabove, be within the range of 1 to 20 grams/liter. If the chloride concentration falls below 1 gram/liter, the coatings will be very low in weight, and will become invisible so that it becomes substantially impossible to distinguish coated from uncoated surfaces. If the amount of chloride ion in the coating solution as initially prepared is allowed to exceed 20 grams/liter, no useful coating will be obtained due to etching of the surfaces. However, it has been found'that after the bath has been in operation for some time, the concentration of chloride ion may exceed 20 grams/liter so long as there are suflicient cations from the class consisting of Zinc and trivalent chromium to combine, stoichiornetrically, with the chloride ion, while simultaneously maintaining the required solution pH and ranges of other essential constituents as taught. As an illustration, one experimental bath has been found to contain upwards of 80 grams/liter of chloride ion and still produced satisfactory coating.

'In connection with the chloride ion content the following should be noted. When using between 1 and 20 grams/liter of chloride ion the coatings produced will tend to be of a light golden to brown color and, somewhat surprisingly, I have discovered that the addition of nitric acid to the coating solutions of this invention tends not only to produce a lighter colored coating of somewhat reduced weight but also to increase the impact resistance, although with a slight sacrificein salt spray corrosion The quantity of nitric acid to be employed for this purpose should be not less than approximately 0.05% by weight. As for the maximum quantity which may be employed, the limit is to be determined, of course, by the pH of the solution which, as above indicated, must be maintained between 0.57 and 3.0. In view of this the quantity of nitric acid which can be utilized must not be more than will lower the pH below 0.57.

The impact test, referred to above, comprises dropping a inch ball upon a test panel under a given inch-pound force. The deformed surface is then examined for loose or cracked paint and ratings in inches of paint failure are reported. The discovery of the effect of the use of nitric acid'is especially surprising in view of applicant's subsequent'discovery that nitric acid, when used alone in place of the hydrochloric acid, fails completely to produce the tween the values of 0.57 and 3.0.

ing solutions of this invention in an amount of at least 1.0 gram/liter, in order to insure obtaining the desired results. Depending upon the particular complex fluoride chosen from the class described above, more or less of this constituent can be utilized. For example, Where either fluosilicate or fluoborate is employed, the upper limit of complex fluoride radical appears to be controlled solely by the limit of solubility of the respective radical in the acid coating solution. However, it should be noted that Where fluotitanate is employed it has been found that use of more than approximately 15 grams/liter of this material tends to somewhat impair the salt spray corrosion resistance of the treated zinc surfaces. However, more than the amount specified of this ingredient has no adverse effect upon the unusually high impact resistance and the bending qualities which are associated with my invention. In other words, if salt spray corrosion resistance is the quality which presents the determining factor in any particular instance, the quantity of fluotitanate should be limited to 15 grams/liter--otherwise, it is unnecessary to place an upper limit upon this ingredient because even larger quantities will produce unusually line results with respect to impact and bending resistance.

So far as the minimum limit of the complex fluoride radical is concerned, it has been found that if the 1.0 gram/liter amount is not attained, the resultant coatings will be non-adherent and will lack the essential paintbonding qualities.

In selecting a salt of the complex fluoride radicals for use in preparing or replenishing the coating solutions of this invention, care must be taken to insure that no alkali or alkaline earth cations be included therewith for the reasons stated above. It is thus essential that this particular component be utilized in the form of its respective acid, or as a zine salt of the complex fluoride radical.

Although a complex fluoride radical from the class described must be included within the coating solutions of this invention, it has been found that simple fluoride ions, as noted hereinabove, must be carefully avoided in the coating solutions of this invention in order to prevent etching of the zinc surfaces being treated. Thus, in selecting a complex fluoride compound suitable for use herein, it is important to insure that such compound contain substantially no simple fluoride salts therein.

In addition to all of the foregoing requirements, success coatings can only be obtained if the solution pH, as measured by standard glass electrodes, is maintained be- If the coating solution pH is permitted to fall below 0.57, a marked etching action, at the expense of coating formnation, will occur. Moreover, if the solution pH is allowed to increase above 3.0, the resultant coatings will possess poor corrosion resistance qualities. A pH range of 0.7 to 1.7 has been found to yield optimum results commensurate with consistently high quality coatings, and is the preferred operconditions, i.e., at 12 C.; or atelevated temperatures up.

to 65 C. However, no noticeable improvement can be attributed to higher operating temperatures, so that such practice is not recommended in the interests of economy.

It has been found, although the reasons therefor are unknown to the inventor, that the quality of the coatings produced in accordance with the teachings of this inven tion may be improved by the addition, to the coating solutions, of an amount of trivalent chromium ions. The amount to be added has been found to be at least 0.25 gram/liter, calculated as Cr. Use of larger quantities of trivalent chromium ions do not appear to have any detrimental effects upon either coating formation or quality, and, where the level of trivalent chromium ion concentration exceeds its solubility limit in the coating bath,

such excess amount is precipitated as insoluble salt from the solution.

The coating solutions of the present invention may be applied to zinc surfaces by any conventional means such as spraying, dipping or roller coating. Roller coating is preferred since it has been found to be particularly adaptable to commercial strip line operations and provides consistently high quality results without the need of auxiliary equipment required in dip or spray operations.

After application of the chromate solution to zinc surfaces the resulting coating may be water rinsed and air dried preparatory to the application of a siccative film. If desired, the chromate conversion coating may be baked after application, whether or not a water rinse was utilized. A preferred operation consists in employing a water rinse followed by air drying, and the subsequent application of a siccative film. The painted metal surface is then baked to cure the decorative organic finish.

Zinc surfaces which are to be coated by the process of this invention should be given a suitable cleaning pretreatment for removal of oils, greases, etc. which are frequently present on such surfaces. However, since this particular operation is well known to the art, and since such cleaning forms no part of the present invention, it need not be described in detail.

By way of example, there are presented below a number of illustrations of solutions and operating conditions which can be used to produce the coatings of this invention, but it is to be understood that these are intended as being merely representative and should not be construed as limitations except as defined in the appended claims.

Example I An aqueous solution was prepared so as to contain:

ZnO grams" 16.5 CrO do 40 H SiF (30% solution) mls 6 Hydrochloric acid, sp. gr. 1.19 mls 6 Water, to make 1 liter.

This solution as prepared had a pH of 1.2 and was applied to clean galvanized steel panels using a 10 second spray application at 48 C. The coatings produced were pale, golden brown in color, and the treated panels were subsequently painted with an acrylic paint and cured at 230 C. for three minutes. After 240 hours of salt spray testing (ASTM-B1176l) the coated and painted panels were examined and found to contain only a trace of corrosion.

clean galvanized steel panels using a second spray contact cycle at 24 C. The panels so treated were then painted with an acrylic paint and after curing as in Example I, were then subjected to salt spray testing in accordance with ASTMB-1l76l. After 240 hours exposure the coated and painted panels were found to be essentially free of corrosion.

Example HI An aqueous solution was prepared containing:

ZnO grams 4.5 CrO do 10.5 ZnTiF do 10 Hydrochloric acid, sp. gr. 1.19 mls 5 Water, to make 1 liter.

The pH of this solution, as prepared, was 1.34. Clean, galvanized steel panels were sprayed, at 26 C. for 15 seconds, with this solution thus resulting in pale, golden brown coatings on the metal surfaces. After the application of an acrylic paint and curing thereof in accordance with the procedure outlined in Example I, the panels were subjected to 336 hours of salt spray testing according to ASTM-B 117-61. No failure was found on any of these panels following this test period. Additional panels coated and painted in accordance with the above procedure were subjected to impact tests with no failure in the coatings being observed.

Example IV A solution was prepared containing:

ZnO 4.5 grams.

CrO 10.6 grams.

Hydrochloric acid 5 mls. (equivalent to 1.8 grams Cl).

Cr (as Cr(NO .9H O) 2 grams.

HgSlFB (30% solution) 6 mls.

Water, to make 1 liter.

The pH of this solution, as prepared, was 1.5. This solution was used to treat a long succession of galvanized steel panels, with replenishment and pH adjustment using nitric acid, being effected, as required, until approximately 1000 ft. of surface area had been coated. The coating cycle was effected at 26 C. using a 30- second dip treatment to produce uniform, iridescent, golden-brown coatings on the galvanized steel. Samples of the coated galvanized panels were painted with an acrylic finish and cured at 230 C. for 3 minutes. Subsequent salt spray corrosion testing (ASTM-B-117-61) showed essentially no failure after 336 hours.

Example V The solution of Example I was applied by means of roller coater apparatus to galvanized strip moving at a lineal speed of 20 feet/minute. Coating was effected at room temperature (24 C.), and the solution applied to the metal surface was allowed to remain thereon for 30 seconds before drying with externally applied heat. An acrylic paint was then applied to the strip and baking was effected at 232 C. Sections of the coated and painted strip were subjected to salt spray corrosion testing (ASTMB11761) and after 240 hours exposure essentially no corrosion was found on the metal surfaces.

From the foregoing examples it is readily apparent that the coating solutions and method of this invention are capable of providing excellent, high quality coatings on zinc surfaces in a minimum of time.

It is within the purview of this invention to provide solid, concentrated formulations of the essential coating constituents for purposes of making up and replenishing the coating solutions of this invention. These solid concentrates must contain from 3.5 to 10% of a zinc salt of a complex fluoride radical selected from the class above described, from 40 to 65% CrO or the equivalent amount of CrO in the form of a zinc salt thereof, and the balance to be supplied by a zinc salt selected from the class consisting essentially of the oxide and the chloride.

The solid, concentrated formulations may in some cases require the addition of hydrochloric acid upon dissolution With water so as to provide the desired initial level of the essential chloride ions.

Typical examples of solid admixtures are as follows:

Example VI Percent ZnSiF 3.6 CrO 65.0 ZnCl 5.4 2110 26.0

Example VII This admixture is dissolved in suflicient dilute hydro chloric acid to make a solution containing by weight of the above composition having a. pH of 1.25.

The present application is in the nature of a continuation-in -part of my earlier application Serial No. 191,705, filed May 2, 19,62, which earlier application is now abandoned.

I claim:

1. An aqueous acid solution consisting essentially of:

(a) at least 3.0 grams/liter of iznc ion (calculated as Zn), v

(b) from 5.0 to 75 grams/liter of hexavalent chromium ion (calculated as CIOg),

(c) from 1 to 20 grams/liter of chloride ion (calculated as Cl), and

(d) at least 1.0 gram/liter of a complex fluoride radical selected from the class consisting essentially of TiFs, BF4 and SlF said solution having a pH between 0.57 and 3.0, and being substantially free of alkali and alkaline earth metal 2. The solution of claim 1 which also contains nitrate 3. The solution of claim 1 which has a pH between 0.7

and 1.7.

4. The solution of claim 1 which also contains at least 0.25 gram/liter of trivalent chromium ion.

5. The solution of claim 2 which also contains at least 0.25 gram/liter of trivalent chromium ion.

6. The method of forming a chromate conversion coating on a zinciferous surface which comprises subjecting the surface to the action of an aqueous acid solution conto 3.0 and the treatment being continued until a chromate conversion coating isformed on'the surface.

7. The process of claim 6 wherein the zinc ion is supplied from the group consisting of zinc, oxide and the chloride, dichromate and fluosilicate salts of zinc.

8. The process of claim 6 wherein the zinc ion is supplied from the group consisting of zinc oxide and the chloride, dichromate and fiuosilicate salts of zinc and wherein the chromium ion is supplied in the form of chromic trioxide (CrO 9. The process of claim 6 wherein the complex fluoride radical is supplied by use of not more than 15 grams/liter of fiuotitanate (Til- 10. The process of claim 9 wherein the complex fluoride radical is supplied from the group consisting of the respective acid and the zinc salt of the complex fluoride acid.

11. The process of claim 6 wherein the Zinc ion is supplied from the group consisting of zinc oxide and the chloride, dichromate and fluosilicate salts of zinc and wherein the zinc content is no greater than the quantity required to satisfy the stoichiometric amount needed to fully satisfy all of the anions present in the coating solution. l

12. The process of claim 6 wherein the solution also contains at least 0.05% by Weight of nitric acid but not suificient to lower the pH below the minimum of 0.57 specified.

13. The method of forming a chromate conversion coating on a zinciferous surface which comprises subjecting the surface to the action of an aqueous acid solution consisting essentially of: V

(a) at least 3.0 grams/liter of zinc ion (calculated as Zn),

(b) from 5.0 to grams/liter of hexavalent chromium ion (calculated as CrO (c) at least 1 gram/liter of chloride ion (calculated as Cl),

(d) at least 1.0 gram/liter of a complex fluoride radical selected from the class which consists of TiF, BR; and SiF and (e) at least .25 gram/ liter of trivalent chromium (calculated as Cr+++),

said chloride ion concentration being no greater than that which is required, stoichiometrically, to combine with cations from the class consisting of zinc and trivalent chromium, said solution being substantially free of alkali and alkaline earth cations and being maintained at a pH of from 0.57 to 3.0 and the treatment being continued until "a chromate conversion coating is formed on the surface.

14. The process of claim 6 wherein the solution also contains at least 0.25 gram/liter of trivalent chromium ion.

15. The process of claim 12 wherein the solution also contains at least 0.25 gram/liter of trivalent chromium ion.

16. The process of claim 13 wherein the solution also contains at least 0.25 gram/liter of trivalent chromium ion.

17. A solid concentrate for use in preparing an aqueous acid coating solution upon dissolution in water, said concentrate consisting essentially of:

(a) 3.5 to 10% of a zinc salt of a complex fluoride radical selected from the class Consisting of T BF4 and SlFe, v Y

(b) from 40 to 65% CrO and 1 (c) the balance zinc salt selected from the class consisting of zinc oxide and zinc chloride.

References Cited in the file of this patent UNITED STATES PATENTS 2,276,353 Thompson Mar. 17, 1942 2,762,731 Hiller Sept. 11, 1956 2,786,002 Vam et al Nov. 19, 1957 2,948,643 Pimbly Aug. 9, 1960 

6. THE METHOD OF FORMING A CHROMATE CONVERSION COATING ON A ZINCIFEROUS SURFACE WHICH COMPRISES SUBJECTING THE SURFACE TO THE ACTION OF AN AQUEOUS ACID SOLUTION CONSISTING ESSENTIALLY OF: (A) AT LEAST 3.0 GRAMS/LITER OF ZINC ION (CALCULATED AS ZN), (B) FROM 5.0 TO 75 GRAMS/LITER OF HEXAVALENT CHROMIUM ION (CALCULATED AS CRO3), (C) FROM 1 TO 20 GRAMS/LITER OF CHLORIDE ION (CALCULATED AS CL), AND (D) AT LEAST 1.0 GRAM/LITER OF A COMPLEX FLUORIDE RADICAL SELECTED FROM THE CLASS WHICH CONSISTS OF TIF6, BF4 AND SIF6, SAID SOLUTION BEING SUBSTANTIALLY FREE OF ALKALI AND ALKALINE EARTH CATIONS AND BEING MAINTAINED AT A PH OF FROM 0.57 TO 3.0 AND THE TREATMENT BEING CONTINUED UNTIL A CHROMATE CONVERSION COATING IS FORMED ON THE SURFACE. 